The invention relates to a retractable or draw type chuck.
There are generally two types of retractable chucks. The first type clamps the side face of a workpiece and thus applies gripping force radially inwardly of the workpiece. The second type clamps an end face of a workpiece and thus applies gripping force in the axial direction of the workpiece. This invention relates to the first type. Atypical conventional chuck of this type is shown in FIG. 4. Some numbered elements in FIG. 4 are not described here but like elements will be described later with reference to the figures showing the embodiment.
The chuck of FIG. 4 includes a housing 3 and three round shafts 5 mounted in the housing and having their proximate ends coupled to a common drawbar (not shown) of the lathe and each carrying a jaw 6 at its distal end remote from the lathe. The shafts 5 are arranged so as to be moved along three equiangularly (120xc2x0) spaced general lines of an imaginary cone coaxial with the drawbar and tapering toward the proximate end of the chuck. Each shaft 5 is formed with a guide groove 5a extending parallel to its axis. In each groove 5a, a guide pin 34 fixed to the housing 3 is received as shown in FIG. 5.
When the shafts 5 are pulled toward the proximate end of the chuck by the drawbar, they are moved along the abovementioned three general lines toward the apex of the imaginary cone all at once. The pins 34 prevent the shafts 5 from rotating about their axes when moved axially. Thus, as the shafts 5 retract, the inner clamp surfaces of the jaws 6 gradually move radially inwardly toward each other until pressed against the outer circumferential surface of the workpiece W as shown in FIG. 4.
On the other hand, the abovementioned second type of chuck (known as a face-clamp chuck; not shown) has three round shafts that are arranged around and parallel to the central axis of the chuck and each carrying a jaw at the distal end thereof When the shafts are pulled toward the proximate end of the chuck by the drawbar, the jaws 6 are moved together with the shafts 5 toward the proximate end of the chuck and pressed against the end face (perpendicular to the central axis of the chuck) of the workpiece to clamp it.
With this type of chuck, each shaft is formed with a guide groove comprising a first portion extending parallel to the central axis of the shaft and a second portion connecting with the proximate end of the first portion and extending obliquely relative to the axis of the shaft toward the proximate end of the shaft. A guide pin fixed to the housing is received in the guide groove of each shaft.
When the shafts are fully extended, the guide pins are at the proximate end of the second portion of the guide grooves, and the jaws are positioned such that their clamping surfaces face away from the central axis of the chuck. In this state, a workpiece is set in the chuck. Then, the shafts are retracted by the drawbar, so that the guide pins move in the second portion of the grooves toward the first portion. Thus, the shafts turn around their own axes so that the clamping surfaces of the jaws all face the central axis of the chuck by the time the guide pins enter the first portion of the grooves. When the shafts are further retracted, the jaws engage the end face of the workpiece to clamp it.
The chuck shown in FIG. 4 has a centering function. But it cannot clamp such a workpiece W as shown in FIG. 1 at its shaft portion W2 with its large-diameter base portion W1 received inside the jaws if the radial length of the base portion W1 is greater than the diameter of the circle defined by the clamping surfaces of the jaws when the shafts are fully extended.
On the other hand, the face-clamp type chuck has no centering function. Thus, precise working is difficult with the face-clamp chuck.
An object of the invention is to provide a chuck having a centering function and capable of clamping a stepped workpiece at its thin portion even if its thick portion has a greater radial length than the maximum clamping diameter of the jaws.
According to the invention, there is provided a retractable chuck comprising a housing adapted to be fastened to a spindle of a lathe, a plurality of round shafts mounted in the housing so as to be rotatable about its central axis and having proximate ends near the lathe adapted to be coupled to a drawbar of the lathe, jaws each fastened to one of the shafts at a distal end thereof remote from the lathe, the shafts being arranged so as to be moved along equiangularly spaced general lines on an imaginary cone coaxial with the housing and tapering toward a proximate end of the chuck near the lathe, each of the shafts being formed with a guide groove having two ends each located at the same axial position as the respective ends of the guide grooves of the other shafts, and guide pins fixed to the housing and each received in one of the guide grooves, each of the grooves comprising a first portion extending parallel to the central axis of the respective shaft, and a second portion extending obliquely with respect to the central axis of the respective shafts from one end of the first portion near the lathe toward the proximate end of the chuck, the guide groove being arranged such that when any of the guide pins is in the first portion of the corresponding guide groove, the corresponding jaw faces the central axis of the chuck.
With this arrangement the workpiece is centered.
The second portions of different shafts may have different forms from each other in length and inclination angle with respect to the first portion.
This makes it possible to turn the shafts and thus jaws not simultaneously but with a time lag. With this arrangement., a longer space is defined between the jaws when the shafts are fully extended than in the arrangement in which the jaws are turned simultaneously in the same direction. Thus, it is possible to set a workpiece having a thick base portion. Also, the jaws will not interfere with each other even if the shaft portion of the workpiece is small in diameter. If all the jaws were turned simultaneously in the same direction, the jaws might interfere with each other if the shaft portion of the workpiece is small in diameter.
Thus it is possible to clamp a stepped workpiece at its small-diameter portion with its base portion received inside the jaws even if the base portion is relatively thick.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: